Vacuum cleaners which utilise cyclonic separators are well known. Examples of such vacuum cleaners are shown in EP 0 042 723, EP 1 370 173 and EP 1 268 076. In general, an airflow in which dirt and dust is entrained enters a first cyclonic separator via a tangential inlet which causes the airflow to follow a spiral or helical path within a collecting chamber so that the dirt and dust is separated from the airflow. Relatively clean air passes out of the chamber whilst the separated dirt and dust is collected therein. In some applications, and as described in EP 0 042 723, the airflow is then passed to a second cyclonic separator which is capable of separating finer dirt and dust than the first cyclonic separator. It has been found useful to position a barrier member, known as a shroud, between the outlet to the first cyclonic separator and the inlet to the second cyclonic separator.
A shroud typically includes a wall having a large number of passageways or through-holes which communicate on their upstream side with the separating chamber of the first cyclonic separator. The through-holes of the shroud thus form the outlet from the first cyclonic separator. In use, some of the dirt and dust not separated by the first cyclonic separator passes through the through-holes in the shroud and into the second cyclonic separator.
A shroud can be useful to prevent larger particles of dirt and dust from passing through the through-holes of the shroud into the second cyclonic separator. However, the nature of a shroud as a barrier member means that a pressure drop will be generated across the shroud. This is because the airflow has to pass through the through-holes of the shroud which acts as a restriction in the airflow path. This may result in high air velocities through the through-holes, potentially leading to unwanted dirt and dust being pulled through the through-holes. Consequently, it is important to provide a sufficiently large surface area of through-holes such that the pressure drop across the shroud is minimised.
The passageways or through-holes in the shroud which form the outlet from the first cyclone separator can take a variety of different forms. EP 0 800 359 discloses a shroud with a plurality of small circular through-holes or passageways formed therein. The circular through-holes of EP 0 800 359 have the advantage that they are simple to manufacture and are dimensioned to prevent larger particles of dirt and dust from passing through the shroud. However, because of their circular shape, they do not provide the largest through-hole to shroud wall ratio per unit area of the shroud.
Alternative arrangements are shown in EP 0 972 573 and GB 2 376 197. In each of these arrangements, a plurality of longitudinal blades is provided around the outlet from the first cyclone separator. The blades define relatively long passageways which have a relatively large cross-sectional area in comparison to through-holes such as, for example, those shown in EP 0 800 359. The passageways have a relatively larger cross-sectional area because they have a reduced number of “partitions” between the passageways. However, due to their larger size, the passageways shown in EP 0 972 573 and GB 2 376 197 allow larger particles of dirt and dust (for example, fluff) to pass through the passageways than the arrangement shown in EP 0 800 359. This may reduce the efficiency at which the vacuum cleaner operates because larger particles of dirt and dust are able to pass into parts of the vacuum cleaner downstream of the first cyclone separator. These arrangements may also be more complicated to manufacture than a shroud comprising circular through-holes as shown in EP 0800 359.